A) Field of the Invention
The present invention relates to a ZnO layer and a semiconductor light emitting device, and more particularly to a ZnO layer suitable for particularly green light emission and a semiconductor light emitting device.
B) Description of the Related Art
Main trends of green light emitting diodes (LED) presently in practical use are InxGa1-xN-containing LED and GaP-containing LED.
An InxGa1-xN-containing LED is formed, for example, on a sapphire substrate. For example, sequentially formed on a (0001) polar plane of a sapphire substrate are a buffer layer, an n-type GaN contact layer, an InGaN emission layer, a p-type GaN clad layer and a p-type GaN contact layer. Green emission can be obtained by changing an In composition of an InxGa1-xN in the InGaN emission layer.
For a GaP-containing LED, green emission (Eg: 2.25 eV to 2.45 eV) can be obtained by using Ga1-xAlxP semiconductor material. However, this LED is not practical because emission is indirect transition and an emission intensity is weak. An emission efficiency can be increased by doping luminescent centers substituting group V and called isoelectronic traps, into GaP. If N is selected as isoelectronic traps, yellow-green emission occurs at a wavelength of 565 nm.
A high emission efficiency at an external quantum efficiency of about 50% can be obtained for a blue InxGa1-xN LED. However, such a high quantum efficiency can only be obtained in a range from ultraviolet in a 360 nm range to blue in a 460 nm range, and in the longer wavelength range, an efficiency is lowered considerably. For example, green in a 530 nm range has an efficiency about a half that of blue. Improving an efficiency of a green LED is important when considering applications to a liquid crystal display using as backlight, LED's of three primary colors: blue of InGaN-containing LED, green of InGaN-containing LED, and red of AlGaInP-containing LED.
An efficiency of a green LED is lowered by a phenomenon which occurs when a mixed crystal In component of InxGa1-xN to be used as the material of the emission layer is increased. By using InxGa1-xN mixed crystal as the material of the emission layer, blue emission at a wavelength of about 470 nm is obtained at an In composition of about 20%, and green emission at a wavelength of about 520 nm is obtained at an In composition of about 35%. However, at an In composition of 20% or higher, phase separation occurs. Green emission characteristics are degraded by phase separation.
In the LED structure described above, since the InxGa1-xN emission layer is thin, this layer grows lattice matching the relatively thick n-type GaN contact layer The InxGa1-xN layer is therefore formed containing strain in crystal. InxGa1-xN having a high mixed crystal In composition has a much larger lattice constant than that of GaN. Therefore, compression strain is contained in crystal of the InxGa1-xN emission layer lattice matching the GaN contact layer. Because of compression strain, a piezo electric field is generated in the InxGa1-xN layer so that carriers (electrons and holes) in crystal are spatially separated and a recombination probability lowers. This results in a lowered internal quantum efficiency of LED. Furthermore, as a device generating a piezo electric field is driven, a drive current increases and emission color shifts to the shorter wavelength side.
FIG. 8 illustrates a band diagram of a double hetero structure of GaAlP/Gap/GaAlP of a GaP-containing LED. Heterojunction of GaP/Ga1-xAlxP has a type II structure. Therefore, a GaP-containing LED having a double hetero structure or a quantum well structure cannot confine carriers in an emission layer. It is therefore difficult to improve the emission characteristics. A conventional light emitting diode has therefore homojunction and its emission efficiency is low. An LED having N isoelectronic traps in GaP has homojunction and an emission efficiency of several %, and renders yellow-green emission at a wavelength of 565 nm to 570 nm.
JP-A-2004-296459 discloses a light emitting device using ZnO doped with Se or S as the material of an emission layer. It describes that this light emitting device renders blue emission at 420 nm (e.g., refer to paragraph [0090]). It also describes that the light emitting device has an emission wavelength of 370 nm to 440 nm depending upon a concentration of Se or S (e.g., refer to paragraphs [0077] and [0091]).